KHOWLEDGE ZONE

He was a king, a commander, and a conqueror. Alexander the Great was so powerful some people called him a god. He was one of the greatest generals in history, and he built a vast empire that extended from the Mediterranean Sea to India.

ALEXANDER’S EARLY LIFE

Alexander was born in Macedonia in 356 bc. His father, King Philip II of Macedonia, hired the famous Greek philosopher Aristotle to tutor young Alexander. In the summer of 336 bc, Philip was murdered by one of his bodyguards. Alexander then became king.

Many people in Macedonia plotted against the young king, but Alexander was shrewd. He quickly ordered the execution of all the conspirators. At the same time, some Greek cities ruled by Macedonia rebelled and others threatened to seek independence. Alexander crushed the rebellions and restored Macedonian rule.

INVASION OF ASIA

Next, in 334 bc, Alexander turned his attention toward the Persian Empire (now Iran) in southwest Asia. Alexander led Macedonian and Greek soldiers to attack Darius III, Persia’s king. Their armies met at Issus in Syria in 333 bc, and fought a fierce battle. Alexander won, and Darius fled.

CONTROL OF EGYPT

Alexander then led his soldiers south, into Egypt. Alexander seized power from the pharaoh, who ruled Egypt on behalf of the Persians. The grateful Egyptians saw Alexander as a person who freed them, and they crowned him pharaoh. At the mouth of the Nile River in northern Egypt, Alexander founded a new city. He named the city Alexandria, and it became a famous center of learning.

CONQUEROR OF PERSIA

In 331 bc, Alexander led his troops back north into Persia. King Darius was eager for revenge. Alexander and Darius fought another great battle, this time at Gaugamela. Once again, Alexander won. The battle at Gaugamela ended centuries of Persian rule in Asia.

Alexander then turned south and conquered other important Persian cities. At Persepolis, he burned down Darius’s palace to show he had conquered the Persian Empire. In 330 bc, Alexander went north to find Darius again. This time, Darius was killed by his own men as he fled.

WORLD EMPIRE

Alexander was a military genius and a great explorer. But he also had a grand ambition. He wanted to rule a world empire where people could live in peace with one another. From 330 to 327 bc, Alexander led his soldiers east, through Afghanistan and into Central Asia. As he travelled, he built more cities. He recruited soldiers, merchants, and scholars from many lands to settle there.

In 326 bc, Alexander turned south, into India. But by then his men were tired and weak. They were far from home in an unknown land. The soldiers rebelled and refused to go farther. Reluctantly, Alexander turned back. By 323 bc, he reached Babylon in Iraq. While there he caught a fever and died at the age of 33. His empire was divided among his generals.

Watch a bolt of lightning flash across the sky. Flip a switch and light up your bedroom. Click the remote and see the TV come on. What do all of these things have in common? Electricity.

WHAT IS ELECTRICITY?

Electricity is a powerful force of nature. Electricity is everywhere in the universe. Electrical forces hold water, metals, and all other kinds of matter together. You can walk and run because electric signals go through your nerves from your brain to your muscles. The signals tell your muscles where to move.

Electricity makes many machines work. Electricity makes bulbs light up and runs motors in saws, fans, hairdryers, and other appliances. The computer you are using works because of electricity.

WHERE DOES ELECTRICITY COME FROM?

Electricity starts with atoms. Atoms are tiny bits of matter much too small for you to see. Everything in the universe is made up of atoms.

Atoms have two main parts: a center or nucleus, and electrons that orbit or go around the nucleus. Electricity comes from electrons. You cannot see electrons and you cannot see electricity. You can see what electricity does because of electric charge and electric energy.

WHAT IS ELECTRIC CHARGE?

Electric charge comes from the parts inside atoms. There are two kinds of electric charge called positive charge and negative charge. Positive charge comes from the nucleus of an atom. Negative charge comes from electrons. Atoms do not normally have any overall charge because their positive and negative charges cancel each other out. Charge comes when electrons move away from an atom.

Positive charge is just the opposite of negative charge. Positive and negative charges pull toward each other. The pull of positive and negative charges makes two kinds of electricity—static electricity and electric current.

WHAT IS STATIC ELECTRICITY?

Did you ever get a shock after walking across a carpet and touching a metal doorknob? That shock came from static electricity. Huge amounts of static electricity cause lightning.

Electrons that move away from their atoms cause static electricity. You can make static electricity by rubbing certain materials together. Run a plastic comb through your hair. Be sure your hair is clean and dry. Electrons jump from your hair to the comb. This gives the comb a negative electric charge. Your hair loses electrons. This gives your hair a positive electric charge. Hold the comb above your head and watch some of your hairs stand on end. Your hair stands on end because the positive and negative charges are pulling toward one another.

Static electricity also causes lightning. The pull of positive and negative charges between clouds and the ground creates a huge spark. The spark is actually the charges moving very quickly toward each other. Lightning can also be caused by opposite charges inside one cloud, between two clouds, or between clouds and the air.

WHAT MAKES LAMPS LIGHT UP?
Electric current makes lamps and all other electric devices work. Electric current is actually electrons moving in a big loop.

Something must give the electrons a push to get them moving. Batteries can start electrons flowing. Batteries are a source of electric energy. A battery, two wires, and a light bulb can make an electric circuit. The current starts flowing from the battery through a wire to the light bulb. The other wire carries the electric current back to the battery. If you cut the wire, the electric current stops. Switches on an electric circuit turn the current on and off. This is how a wall switch works to turn lights on and off in your home.

The electric energy in your home does not come from batteries. You plug appliances into electric outlets in your walls. The electric energy in the outlets comes from electric power plants.

HOW DO POWER PLANTS WORK?

Huge electric power plants generate or make electricity. Steam or falling water in dams make big machines called turbines turn. The turbine drives another machine called an electric generator. The generator makes electricity.

Long power lines carry electricity from power plants to your home. Wires inside your home bring the electric energy to light bulbs, TVs, microwaves, and your computer.

WHO DISCOVERED ELECTRICITY?

For thousands of years people knew that a material called amber mysteriously pulled on some materials. The ancient Greeks called amber elektron. Scientists in Europe in the 1600s and early 1700s called the materials that amber attracted electrics.

Benjamin Franklin, an American printer, patriot, and inventor, experimented with electricity. He thought lightning and electricity were the same thing. He did a dangerous experiment in the mid-1700s to find out. Franklin flew a kite during a thunderstorm. He attached a metal key to the kite string. An electric charge ran down the wet kite string to the key. The charge made a spark when it hit the key. This showed Franklin that lightning was electricity. He was lucky he was not killed.

Many other scientists have experimented with electricity since Benjamin Franklin. They learned how to make electricity with batteries. They found that electricity would go through wires. An American inventor named Thomas Alva Edison invented many things that use electricity, including the electric light bulb.

People ran and hid when they saw Viking ships coming. “From the fury of the Northmen, good Lord deliver us!” they prayed. The Vikings were daring, skillful sailors and frightening warriors. For 300 years, from ad 800 to 1100, they terrorized much of northern Europe.

HARSH HOMELAND

The Vikings came from Scandinavia, an area in northern Europe that now includes the countries of Norway, Denmark, and Sweden. The climate there is harsh, with bitter winters and short summers. Viking families struggled to survive. They grew oats and barley during the short growing season. They also hunted deer, bear, and wolves in the woods and caught fish, seals, and whales from the ocean.

RAIDERS AND SETTLERS

Viking warriors sailed in fast, sleek drakkar (dragon ships). They had a single sail but the warriors also rowed them with oars.

The Vikings sailed south to attack villages and churches around the coasts of Europe. They seized valuables and kidnapped men, women, and children to sell as slaves. The Vikings demanded danegeld (money for protection) from towns and threatened to attack again if they weren’t paid. Boatloads of Vikings landed in Scotland, Ireland, England, and France. They killed local rulers and set up their own kingdoms. Vikings also took control of parts of Russia and the Ukraine.

Brave Viking adventurers headed west across the Atlantic to settle in Iceland and Greenland. About ad 1000, they became the first Europeans to land in North America. They called the new land Vinland and built a settlement there, but the settlement did not survive.

SKILLED AND CIVILIZED

Wherever they traveled, the Vikings brought violence. But they also brought their own rich culture. They were expert craftspeople, carving wood, stone, bone, and antlers into useful everyday objects. They forged sharp iron axes, swords, and spears. They made the best ships in Europe, and the best jewelry. They were also shrewd traders. They traveled long distances to sell amber, furs, and slaves in such cities as Constantinople (now Istanbul, Turkey) and Baghdad (in Iraq).

Viking people were proud of their highly developed society. They held community meetings, called things, to punish criminals and make laws. They set up carved memorial stones to honor respected leaders. Viking poets memorized stories and histories of brave deeds.

Viking traditions survive throughout northern Europe, and Viking tales of gods, heroes, and monsters still inspire stirring works of art.

They can cut metal. They can cut glass. They are billions of years old and form deep within Earth. Some are so rare and beautiful that people will pay a fortune to get one! They are diamonds.

WHAT ARE DIAMONDS?

Diamonds are made of carbon, a chemical element. That’s the same stuff that’s in the tip of a pencil. But isn’t carbon soft, not hard?

Pencil carbon is so soft that it rubs off on paper. But diamond carbon is so hard it can cut almost anything. In fact, diamonds are the hardest things in nature. What makes the difference? The atoms inside a diamond have a special crystal shape. This makes them very hard.

Some diamonds can sparkle brilliantly. Such diamonds are rare. They are precious and valuable, and people wear them as jewelry. But most diamonds are small and contain flaws. These are worth much less.

HOW DO DIAMONDS FORM?

Big diamonds form deep under the ground. The heat and pressure at great depths melts carbon-rich rocks. Diamonds form out of this molten (melted) rock. The diamonds are later brought to Earth’s surface by currents of molten rock. The entire process can take billions of years!

Smaller diamonds can form closer to Earth’s surface. Movements of the planet’s crust lift these stones up from the depths.

WHAT CAN DIAMONDS DO?

You can scratch or cut just about anything with a diamond—except another diamond. On a scale of hardness from 1 to 10 with 10 as the hardest, diamonds are a 10.

Because they are so hard, diamonds are great for drills, saws, and other cutting tools. That makes them very useful in mining and manufacturing.

SYNTHETIC AND IMITATION DIAMONDS

Are there any shortcuts to making diamonds? Actually, there are. People have learned how to make artificial diamonds and other gems. Diamonds made by people are called synthetic diamonds.

Synthetic diamonds look and act like the real thing. They’re as hard as natural ones. In fact, most diamonds used for industrial purposes are synthetic.

Imitation diamonds aren’t diamonds at all. They’re usually made from inexpensive materials that sparkle brightly, such as quartz or glass. But they aren’t as hard as natural or synthetic diamonds. Imitation diamonds are used mostly in jewelry.

WHY ARE DIAMONDS CUT?

Diamonds found in the ground are rough. They don’t shine like the gems in jewelry stores. They look greasy or dirty. So how do you make diamonds really sparkle? You cut them.

Cutting a diamond gives the stone edges that bring out the shine. Diamond cutters trim away flaws such as cracks or cloudy spots.

It takes careful planning to cut a diamond. The goal is to make the diamond as large and as valuable as possible. Even so, a cutter slices away about half of a diamond’s original size. After polishing, the cut diamond is ready to sell.

WHAT MAKES DIAMONDS WORTH SO MUCH?

Diamonds are judged by the “4 Cs.” These are color, clarity, carat weight, and cut.

Color: When diamonds form, small amounts of minerals get inside them. This changes their color. Colorless stones are very desirable. Yellowish or brownish ones are not.

Clarity: Can you see flaws in the diamond? A diamond with great clarity has no visible flaws. The clearer a stone is, the greater its value.

Carat weight: The heavier a diamond is, the better. Because large, heavy diamonds are less common, they are more valuable. A diamond that weighs 2 carats or more is usually considered large.

Cut: Diamond cutters decide on the shape and proportion of a stone. An attractive cut makes a diamond more desirable.

HOW BIG CAN DIAMONDS GET?

The largest natural diamond ever found is the famous Cullinan diamond. It was discovered in South Africa in 1905. The Cullinan tipped the scales at an amazing 3,106 carats!

What do you do with a diamond that big? The Cullinan was cut into 105 different gems. The largest of these is called the Star of Africa. It is the largest cut diamond in the world. Today, the Star of Africa sits on the end of the British royal scepter.

Take a really deep breath. Feel how your chest gets bigger and bigger. Your chest gets bigger because your lungs are filling up with air. You cannot see air, but air is all around you. You can feel it when the wind blows.

Earth’s atmosphere is made of air. An atmosphere is made up of the gases that surround a planet.

WHAT IS AIR?

Air is a mixture of several different gases. The main gases in air are nitrogen, oxygen, and argon. Air also contains smaller amounts of hydrogen, carbon dioxide, water vapor, helium, and other gases. Oxygen is the most important gas for animals. Animals must breathe oxygen in order to live.

Carbon dioxide is the most important gas for plants. Plants use carbon dioxide and sunlight to make food. Plants give off oxygen. Animals turn the oxygen back into carbon dioxide when they breathe.

TAKING AIR WITH YOU

You can go to places where there is no air. There is no air underwater, but you can dive underwater. You can stay underwater a short time just by holding your breath. Air tanks let you stay underwater for a long time. Scuba divers wear tanks on their backs. The tanks are filled with gases that make up air. The divers breathe the gases through hoses.

There is less and less air the higher up you go. People gasp for breath at the tops of tall mountains. Airplanes must carry air. Once the airplane gets up high, air is pumped into the cabin where passengers sit. Astronauts have to take all the air they need with them—there’s no air in space!

They called him “the man with a million stories.” People flocked to Marco Polo’s home to hear him tell exciting tales about his travels in distant lands.

Marco Polo won fame for his journeys across Asia. He wrote a book about his travels that became one of the most famous travel guides in history.

EARLY LIFE

Marco Polo was born in 1254 to a family of merchants. His home was Venice, Italy. Venetian merchants bought and sold valuable Chinese goods, including precious silk cloth. Such goods were brought to Europe along an ancient route known as the Silk Road. The merchants also used the route to travel east on trading missions.

Marco’s mother died when he was a young boy. His family taught him to be a merchant. He learned how to read, write, calculate, and use foreign money.

In 1269, Marco’s father and uncle, Niccolò and Maffeo, returned to Venice after visiting China. In China, they had met the Mongol conqueror Kublai Khan. The khan invited the Polos to return. He asked them to bring Christian scholars to explain the Christian religion to him.

TRIP TO CHINA

In 1271, Niccolò and Maffeo set out for China again. Marco, then 17 years old, joined his father and uncle for the trip. Two priests also traveled with the Polos. But the route was dangerous, and the priests soon turned back.

It took the Polos four difficult years to reach China. The journey led across deserts and high mountains. They passed through wild countryside where bandits lurked, ready to rob and kill. They braved heat and cold, floods, deep snowdrifts, and blinding sandstorms. At last they reached the summer palace of Kublai Khan at Shangdu.

The khan welcomed the Polos warmly. He offered Marco a job. Marco accepted, and the Polos lived in China for the next 17 years. Marco traveled on many special missions across the khan’s kingdom and to distant lands. When Marco returned from his missions, he told the khan vivid stories about the people and lands he visited.

Over time, the Polos worried that Kublai Khan would not allow them to leave. Several times they had asked the khan for permission to return to Europe. But the khan enjoyed his visitors so much that he would not grant their wish. Finally, the khan changed his mind.

RETURN TO VENICE

In 1292, Kublai Khan asked Marco to escort a Mongol princess to Persia. The Polos traveled by ship from China to the Persian Gulf. Then the Polos headed for Venice, finally reaching home in 1295. Marco had been away so long that nobody recognized him!

We know about Marco’s travels because, in 1298, he became a prisoner of war. He shared his cell with a writer named Rustichello, who helped Marco turn his stories into a book. Rustichello added some details of his own. But much of Marco’s book seems to be true!

Marco Polo returned to Venice after his release from prison. He died in 1324. But his book remained popular for centuries. Merchants, mapmakers, and explorers all looked to the book for information about Asian lands. Even the navigator Christopher Columbus owned a copy!

Did you ever look at the Moon and think you could see a face? Sometimes dark spots on the Moon look like eyes, a nose, and a mouth. People used to talk about “the man in the Moon.” They would joke about the Moon being made of cheese with holes in it.

The Moon is the second brightest thing in our sky, after the Sun. The Moon doesn’t make its own light. Light rays from the Sun bounce off it and make it shine. The Moon is closer to Earth than any other body in our solar system.

WHAT’S ON THE MOON?


In the 1600s, the famous Italian scientist Galileo was the first person to look at the Moon through a telescope. He saw dark spots that he thought were oceans. He called them maria, the Latin word for “seas.” Galileo thought the light areas were large landmasses called continents.

Today, we know a lot more about the Moon. We know that nothing lives on the Moon, and there are no oceans. The maria are dry, flat plains covered with rocks. The Moon is the only place in space that human beings have visited.

TOUCHING THE MOON

The first astronauts landed on the Moon in 1969. They traveled in a United States spacecraft named Apollo 11. The astronauts set up experiments on the Moon and brought some moon rocks back to Earth. Later, five more Apollo missions explored different parts of the Moon. The astronauts on these missions brought back more rocks and soil.

Scientists learned many things about the Moon from the Apollo space missions. They also learned from other spacecraft that orbited (went around) the Moon. Some of these spacecraft sent robot landers down to the surface of the Moon.

SPACE ROCKS AND CRATERS

The dry, gray Moon might seem like a boring place now. But you should have seen it several billion years ago.

Many times over the past two or three billion years, chunks of rock and ice have come whizzing toward the Moon. The space rocks and ice are asteroids and comets. They slam into the Moon’s surface. The biggest ones came just after Earth and the other planets were formed. When they hit the Moon, these large objects threw up tons of rock and dust. There are billions of big and small pits on the Moon made by the space rocks. These pits are called craters.

ANCIENT VOLCANOES

If you went to the Moon, you’d see the dark-colored maria. Scientists think the dark gray rock is lava (melted rock). They believe that billions of years ago, red-hot rock gushed up from volcanoes on the Moon. The lava flowed over the Moon’s surface. It filled in low places, including some of the big craters. Then the lava cooled to make the Moon’s gray rocks.

The lava also left round hills on the Moon called domes and carved grooves called rilles.

ROUGH HIGHLANDS

There are rough and mountainous places all over the Moon. Scientists call these places highlands.

There are highlands on the far side of the Moon but almost no maria. Only one side of the Moon faces Earth, so you can never see the far side of the Moon. Scientists learned what the far side looks like from pictures taken by orbiting spacecraft.

HOT DAYS AND COLD NIGHTS

The astronauts who walked on the Moon had to wear big space suits. The space suits provided air for the astronauts to breathe, because there is no air on the Moon. The suits also kept the astronauts cool during hot Moon days and warm during cold Moon nights.

With no atmosphere to protect it, Moon temperatures can be very high and very low. It can be 261° Fahrenheit (127° Celsius) at noon during a Moon day—hotter than boiling water! It can be as cold as -279° Fahrenheit (-173° Celsius) on a Moon night. Days and nights on the Moon each last about two weeks.

Days and nights are long because the Moon turns very slowly. It takes the Moon about 27 days to make one turn. Earth turns once every 24 hours.

ICE ON THE MOON?

There is no water on the Moon, but scientists think that there may be ice. Two spacecraft in the 1990s saw signs of the ice. If there is ice on the Moon, it could help future explorers stay there longer.

The signs of ice were found in deep craters at the north and south poles of the Moon. Because these craters are always in shadow, it stays very cold there—about -364° Fahrenheit (-220° Celsius).

THE MOON FROM EARTH

The Moon always seems to change shape. Sometimes it looks like a round ball in the sky. Sometimes it is a thin sliver. But the Moon does not really change shape. What happens to it?

The Moon reflects light from the Sun. How you see the reflected sunlight depends on where the Moon is. The Moon orbits (goes around) Earth. Sometimes it is between the Sun and Earth, and you can’t see any reflected sunlight. This is called the new moon.

Sometimes Earth is between the Moon and the Sun. You can see all of the reflected sunlight. The Moon looks round. This is called a full moon.

The rest of the time, you see only part of the reflected sunlight from the Moon. The reflected sunlight looks like slivers of Moon. It takes about 27 days to go from a new moon to a full moon and back to a new moon again.

WHERE THE MOON CAME FROM

No one knows for sure how the Moon was formed. By testing moon rocks, scientists have learned that the Moon is about 4.6 billion years old. This is the same age as the solar system.

Scientists think that at that time something as big as a planet crashed into Earth. The collision blasted huge pieces of Earth into space. Some of the pieces came together to make the Moon.

Scientists continue to study moon rocks for clues. There is still much to learn about the Moon.